Methods and devices for determining precoder parameters in a wireless communication network

ABSTRACT

A method and a device for determining parameters of a precoder in a wireless communication system are disclosed. According to one aspect, a method includes selecting a subset of beams corresponding to a plurality of orthogonal beams; obtaining power levels of the selected subset of beams for generating a first factor of the precoder and obtaining phases of the selected subset of beams for generating a second factor, wherein the first factor and the second factor are part of the parameters of the precoder.

CROSS-REFERENCE TO RELATED APPLICATIONS

This non-provisional patent application claims priority based upon:

-   -   1) the prior U.S. provisional patent application entitled        “FACTORIZED PRECODER STRUCTURE FOR MULTI-BEAM PRECODER        CODEBOOKS”, application No. 62/316,820, filed Apr. 1, 2016, in        the names of Sebastian FAXER and Svante BERGMAN;    -   2) the prior U.S. provisional patent application entitled “BEAM        SPACE ROTATION FEEDBACK FOR MULTI-BEAM PRECODER CODEBOOKS”,        application No. 62/315,972, filed Mar. 31, 2016, in the names of        Sebastian FAXER and Svante BERGMAN;    -   3) the prior U.S. provisional patent application entitled        “FREQUENCY PARAMETRIZATION OF BEAM CO-PHASING FOR MULTI-BEAM        PRECODER CODEBOOKS”, application No. 62/316,857, filed Apr. 1,        2016, in the names of Sebastian FAXER and Svante BERGMAN.

FIELD

The present disclosure relates to wireless communications, and inparticular, to a factorized precoder structure for multi-beam precodercodebooks.

BACKGROUND

Multi-antenna techniques can significantly increase the data rates andreliability of a wireless communication system. The performance isparticularly improved if both the transmitter and the receiver areequipped with multiple antennas, which results in a multiple-inputmultiple-output (MIMO) communication channel. Such systems and/orrelated techniques are commonly referred to as MIMO.

The Long Term Evolution (LTE) standard is currently evolving withenhanced MIMO support. A component in LTE is the support of MIMO antennadeployments and MIMO related techniques. Currently LTE-Advanced supportsan 8-layer spatial multiplexing mode for 8 transmit (Tx) antennas withchannel dependent precoding. The spatial multiplexing mode is aimed forhigh data rates in favorable channel conditions. An illustration of thespatial multiplexing operation 100 is provided in FIG. 1, where thereare N_(T) antenna 110 ports and N_(T) inverse fast Fourier transformers(IFFTs) 120.

As seen, the information carrying symbol vector s 130 is multiplied byan N_(T)×r precoder matrix W 140, which serves to distribute thetransmit energy in a subspace of the N_(T) (corresponding to N_(T)antenna ports) dimensional vector space. The precoder matrix W 140 istypically selected from a codebook of possible precoder matrices, andtypically indicated by means of a precoder matrix indicator (PMI), whichspecifies a unique precoder matrix in the codebook for a given number ofsymbol streams. The r symbols in s 130 each correspond to a layer 150and r is referred to as the transmission rank. In this way, spatialmultiplexing is achieved since multiple symbols can be transmittedsimultaneously over the same time/frequency resource element (TFRE). Thenumber of symbols r is typically adapted to suit the current channelproperties.

LTE uses Orthogonal Frequency Division Multiplexing (OFDM) in thedownlink (and Discrete Fourier Transform (DFT) precoded OFDM in theuplink) and hence the received N_(R)×1 vector y_(n) for a certain TFREon subcarrier n (or alternatively data TFRE number n) is thus modeled by

y _(n) =H _(n) Ws _(n) +e _(n)  Equation 1

where e_(n) is a noise/interference vector obtained as realizations of arandom process, and N_(R) is the number of receive antennas. Theprecoder W can be a wideband precoder, which is constant over frequency,or frequency selective.

The precoder matrix W is often chosen to match the characteristics ofthe N_(R)×N_(T) MIMO channel matrix H_(n), resulting in so-calledchannel dependent precoding. This is also commonly referred to asclosed-loop precoding and essentially strives for focusing the transmitenergy into a subspace which is strong in the sense of conveying much ofthe transmitted energy to the wireless device. In addition, the precodermatrix may also be selected to strive for orthogonalizing the channel,meaning that after proper linear equalization at the wireless device,the inter-layer interference is reduced.

One example method for a wireless device to select a precoder matrix Wcan be to select the W_(k) that maximizes the Frobenius norm of thehypothesized equivalent channel:

$\begin{matrix}{\max\limits_{k}{{{\hat{H}}_{n}W_{k}}}_{F}^{2}} & {{Equation}\mspace{14mu} 2}\end{matrix}$

Where Ĥ_(n) is a channel estimate, possibly derived from Channel StateInformation-Reference Signal (CSI-RS) as described below:W_(k) is a hypothesized precoder matrix with index k; andĤ_(n)W_(k) is the hypothesized equivalent channel.

In closed-loop precoding for the LTE downlink, the wireless devicetransmits, based on channel measurements in the forward link (downlink),recommendations to the base station, e.g., eNodeB (eNB), of a suitableprecoder to use. The base station configures the wireless device toprovide feedback according to the wireless device's transmission mode,and may transmit CSI-RS and configure the wireless device to usemeasurements of CSI-RS to feedback recommended precoding matrices thatthe wireless device selects from a codebook. A single precoder that issupposed to cover a large bandwidth (wideband precoding) may be fedback. It may also be beneficial to match the frequency variations of thechannel and instead feedback a frequency-selective precoding report,e.g., several precoders, one per subband. This is an example of the moregeneral case of channel state information (CSI) feedback, which alsoencompasses feeding back other information than recommended precoders toassist the base station in subsequent transmissions to the wirelessdevice. Such other information may include channel quality indicators(CQIs) as well as transmission rank indicator (RI).

Given the CSI feedback from the wireless device, the base stationdetermines the transmission parameters it wishes to use to transmit tothe wireless device, including the precoding matrix, transmission rank,and modulation and coding scheme (MCS). These transmission parametersmay differ from the recommendations the wireless device makes.Therefore, a rank indicator and MCS may be signaled in downlink controlinformation (DCI), and the preceding matrix can be signaled in DCI orthe base station can transmit a demodulation reference signal from whichthe equivalent channel can be measured. The transmission rank, and thusthe number of spatially multiplexed layers, is reflected in the numberof columns of the precoder W. For efficient performance, it is importantthat a transmission rank that matches the channel properties isselected.

In LTE Release-10 (Rel-10), a new reference symbol sequence wasintroduced for the intent to estimate downlink channel stateinformation, the CSI-RS. The CSI-RS provides several advantages overbasing the CSI feedback on the common reference symbols (CRS) which wereused, for that purpose, in previous releases. First, the CSI-RS is notused for demodulation of the data signal, and thus does not require thesame density (i.e., the overhead of the CSI-RS is substantially less).Secondly, CSI-RS provides a much more flexible means to configure CSIfeedback measurements (e.g., which CSI-RS resource to measure on can beconfigured in a wireless device specific manner).

By measuring a CSI-RS transmitted from the base station, a wirelessdevice can estimate the effective channel the CSI-RS is traversingincluding the radio propagation channel and antenna gains. In moremathematical rigor, this implies that if a known CSI-RS signal x istransmitted, a wireless device can estimate the coupling between thetransmitted signal and the received signal (i.e., the effectivechannel). Hence, if no virtualization is performed in the transmission,the received signal y can be expressed as

Y=Hx+e  Equation 3

and the wireless device can estimate the effective channel H

Up to eight CSI-RS ports can be configured in LTE Rel-10, that is, thewireless device can estimate the channel from up to eight transmitantennas.

Related to CSI-RS is the concept of zero-power CSI-RS resources (alsoknown as a muted CSI-RS) that are configured just as regular CSI-RSresources, so that a wireless device knows that the data transmission ismapped around those resources. The intent of the zero-power CSI-RSresources is to enable the network to mute the transmission on thecorresponding resources, in order to boost the Signal to Interferenceplus Noise Ratio (SINR) of a corresponding non-zero power CSI-RS,possibly transmitted in a neighbor cell/transmission point. For Release11 (Rel-11) of LTE, a special zero-power CSI-RS was introduced that awireless device is mandated to use for measuring interference plusnoise. A wireless device can assume that the transmission points (TPs)of interest are not transmitting on the zero-power CSI-RS resource, andthe received power can therefore be used as a measure of theinterference plus noise.

Based on a specified CSI-RS resource and on an interference measurementconfiguration (e.g., a zero-power CSI-RS resource), the wireless devicecan estimate the effective channel and noise plus interference, andconsequently also determine the rank, precoding matrix, and MCS torecommend to best match the particular channel.

Some installations are equipped with two dimensional antenna arrays andsome of the presented embodiments use such antennas. Such antenna arraysmay be (partly) described by the number of antenna columns correspondingto the horizontal dimension N_(h), the number of antenna rowscorresponding to the vertical dimension N_(v) and the number ofdimensions corresponding to different polarizations N_(p). The totalnumber of antennas is thus N=N_(h)N_(v)N_(p). It should be pointed outthat the concept of an antenna is non-limiting in the sense that it canrefer to any virtualization (e.g., linear mapping) of the physicalantenna elements. For example, pairs of physical sub-elements could befed the same signal, and hence share the same virtualized antenna port.

An example of a 4×4 array with cross-polarized antenna elements 200 isshown in FIG. 2, where the horizontal dimension “1” represents N_(h) andthe vertical dimension “m” represents the N_(v).

Precoding may be interpreted as multiplying the signal with differentbeamforming weights for each antenna prior to transmission. A typicalapproach is to tailor the precoder to the antenna form factor, i.e.,taking into account N_(h), N_(v) and N_(p) when designing the precodercodebook.

A common type of precoding is to use a DFT-precoder, where the precodervector used to precede a single-layer transmission using asingle-polarized uniform linear array (ULA) with N antennas is definedas

${{w_{1\; D}(k)} = {\frac{1}{\sqrt{N}}\begin{bmatrix}e^{j\; 2\; {\pi \cdot 0 \cdot \frac{k}{QN}}} \\e^{j\; 2\; {\pi \cdot 1 \cdot \frac{k}{QN}}} \\\vdots \\e^{j\; 2\; {\pi \cdot {({N - 1})} \cdot \frac{k}{QN}}}\end{bmatrix}}},$

where k=0, 1, . . . QN−1 is the precoder index and Q is an integeroversampling factor. A corresponding precoder vector for atwo-dimensional uniform planar array (UPA) can be created by taking theKronecker product of two precoder vectors as w_(2D)(k,l)=w_(1D)(k)⊗w_(1D)(l). Extending the precoder for a dual-polarized UPAmay then be done as

${w_{{2\; D},{DP}}\left( {k,l,\varphi} \right)} = {{\begin{bmatrix}1 \\e^{j\; \varphi}\end{bmatrix} \otimes {w_{2\; D}\left( {k,l} \right)}} = {\quad{{\begin{bmatrix}{w_{2\; D}\left( {k,l} \right)} \\{e^{j\; \varphi}{w_{2\; D}\left( {k,l} \right)}}\end{bmatrix} = {\begin{bmatrix}{w_{2\; D}\left( {k,l} \right)} & 0 \\0 & {w_{2\; D}\left( {k,l} \right)}\end{bmatrix}\begin{bmatrix}1 \\e^{j\; \varphi}\end{bmatrix}}},}}}$

where e^(jϕ) is a co-phasing factor that may for instance be selectedfrom the QPSK alphabet

$\varphi \in {\left\{ {0,\frac{\pi}{2},\pi,\frac{3\; \pi}{2}} \right\}.}$

A precoder matrix W_(2D,DP) for multi-layer transmission may be createdby appending columns of DFT precoder vectors as

W _(2D,DP) =[w _(2D,DP)(k ₁ ,l ₁,ϕ₁)w _(2D,DP)(k ₂ ,l ₂,ϕ₂) . . . w_(2D,DP)(k _(R) ,l _(R),ϕ_(R))],

where R is the number of transmission layers, i.e., the transmissionrank. In a common special case for a rank-2 DFT precoder, k₁=k₂=k andl₁=l₂=l, meaning that

$W_{{2\; D},{DP}} = {\begin{bmatrix}{w_{{2\; D},{DP}}\left( {k,l,\varphi_{1}} \right)} & {w_{{2\; D},{DP}}\left( {k,l,\varphi_{2}} \right)}\end{bmatrix} = {\left\lbrack \begin{matrix}{w_{2\; D}\left( {k,l} \right)} & 0 \\0 & {w_{2\; D}\left( {k,l} \right)}\end{matrix} \right\rbrack {\quad{\left\lbrack \begin{matrix}1 & 1 \\e^{j\; \varphi_{1}} & e^{j\; \varphi_{2}}\end{matrix} \right\rbrack.}}}}$

With multi-user MIMO, two or more users in the same cell areco-scheduled on the same time-frequency resource. That is, two or moreindependent data streams are transmitted to different wireless devicesat the same time, and the spatial domain is used to separate therespective streams. By transmitting several streams simultaneously, thecapacity of the system can be increased. This however, comes at the costof reducing the SINR per stream, as the power has to be shared betweenstreams and the streams will cause interference to each-other.

When increasing the antenna array size, the increased beamforming gainwill lead to higher SINR, however, as the user throughput depends onlylogarithmically on the SINR (for large SINRs), it is instead beneficialto trade the gains in SINR for a multiplexing gain, which increaseslinearly with the number of multiplexed users.

Accurate CSI is required in order to perform appropriate nullformingbetween co-scheduled users. In the current LTE Release 13 (Rel-13)standard, no special CSI mode for MU-MIMO exists and thus, MU-MIMOscheduling and precoder construction has to be based on the existing CSIreporting designed for single-user MIMO (that is, a PMI indicating aDFT-based precoder, a RI and a CQI). This may prove quite challengingfor MU-MIMO, as the reported precoder only contains information aboutthe strongest channel direction for a user and may thus not containenough information to do proper nullforming, which may lead to a largeamount of interference between co-scheduled users, reducing the benefitof MU-MIMO.

A multi-beam precoder may be defined as a linear combination of severalDFT precoder vectors as

${w_{MB} = {\sum\limits_{i}\; {c_{i} \cdot {w_{{2\; D},{DP}}\left( {k_{i},l_{i},\varphi_{i}} \right)}}}},$

where {c_(i)} may be general complex coefficients. Such a multi-beamprecoder may more accurately describe the wireless device's channel andmay thus bring an additional performance benefit compared to a DFTprecoder, especially for MU-MIMO where rich channel knowledge isdesirable in order to perform nullforming between co-scheduled wirelessdevices.

Existing solutions for MU-MIMO based on implicit CSI reports withDFT-based precoders have problems with accurately estimating andreducing the interference between co-scheduled users, leading to poorMU-MIMO performance.

Multi-beam precoder schemes may lead to better MU-MIMO performance, butat the cost of increased CSI feedback overhead and wireless deviceprecoder search complexity.

SUMMARY

Some embodiments advantageously provide a method and device fordetermining parameters of a precoder in a wireless communication system.According to a first aspect, the method includes selecting a subset ofbeams from a plurality of orthogonal beams, obtaining power levels ofthe selected subset of beams for generating a first factor of theprecoder and obtaining phases of the selected subset of beams forgenerating a second factor of the precoder; wherein the first factor andsecond factor are part of the parameters of the precoder.

According to a second aspect, there is provided a wireless device fordetermining parameters of a precoder in a wireless communication system.The wireless device includes processing circuitry including a memory anda processor. The processing circuitry is configured to: select a subsetof beams from a plurality of orthogonal beams; obtain power levels ofthe selected subset of beams for generating a first factor of theprecoder; and obtain phases of the selected subset of beams forgenerating a second factor of the precoder, wherein the first factor andthe second factor are part of the parameters of the precoder.

According to a third aspect, there is provided a method for sendingparameters of a precoder to a network node in a wireless communicationsystem. The method comprises sending to the network node, a subset ofbeams selected from a plurality of orthogonal beams and power levels ofthe selected subset of beams, for a first frequency granularity, andsending, to the network node, phases of the selected subset of beams,for a second frequency granularity, wherein the selected subset ofbeams, the power levels and the phases of the selected subset of beamsare part of the parameters of the precoder.

According to a fourth aspect, there is provided a wireless device forsending parameters of a precoder to a network node in a wirelesscommunication system. The wireless device comprises a processingcircuitry including a processor and a memory. The processing circuitryis configured to cause the wireless device to: send to the network node,a subset of beams selected from a plurality of orthogonal beams andpower levels of the selected subset of beams, for a first frequencygranularity; and send, to the network node, phases of the selectedsubset of beams, for a second frequency granularity, wherein theselected subset of beams, the power levels and the phases of theselected subset of beams are part of the parameters of the precoder.

According to a fifth aspect, there is provided a method for determiningtransmission parameters for a wireless device, in a wirelesscommunication system. The method comprises: responsive to transmittingreference signals to the wireless device, receiving precoder parameterswhich include a subset of beams selected from a plurality of orthogonalbeams and power levels of the selected subset of beams for a firstfrequency granularity, and phases of the selected subset of beams for asecond frequency granularity; and determining the transmissionparameters based on the received precoder parameters.

According to a sixth aspect, there is provided a network node fordetermining transmission parameters for a wireless device, in a wirelesscommunication system. The network node comprises a processing circuitryincluding a processor and a memory. The processing circuitry isconfigured to cause the network node to: responsive to transmittingreference signals to the wireless device, receive precoder parameterswhich include a subset of beams selected from a plurality of orthogonalbeams and power levels of the selected subset of beams for a firstfrequency granularity, and phases of the selected subset of beams for asecond frequency granularity; and determine the transmission parametersbased on the received precoder parameters.

According to a seventh aspect, there is provided a method fordetermining transmission parameters for a wireless device, in a wirelesscommunication system. The method comprises: in response to transmittingreference signals, receiving precoder parameters which include a subsetof beams selected from a plurality of orthogonal beams, a first factorassociated with power levels of the selected subset of beams, and asecond factor associated with phases of the selected subset of beams;and determining the transmission parameters based on the receivedprecoder parameters.

According to an eighth aspect, there is provided a network node fordetermining transmission parameters for a wireless device, in a wirelesscommunication. The network node comprises a processing circuitryconfigured to cause the network node to: in response to transmittingreference signals, receive precoder parameters which include a subset ofbeams selected from a plurality of orthogonal beams, a first factorassociated with power levels of the selected subset of beams, and asecond factor associated with phases of the selected subset of beams;and determine the transmission parameters based on the received precoderparameters.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present embodiments, and theattendant advantages and features thereof, will be more readilyunderstood by reference to the following detailed description whenconsidered in conjunction with the accompanying drawings wherein:

FIG. 1 is a block diagram of a known transmitter implementing digitalbeam forming;

FIG. 2 is an illustration of a planar array of co-polarized antennaelements;

FIG. 3 illustrates a schematic diagram of a wireless communicationsystem/network;

FIG. 4A-4D are graphs of an angular spread of a channel for fourdifferent beam space rotation factors;

FIG. 5 is a signaling diagram between a wireless device and a networknode for exchanging precoder information;

FIG. 6 is a flow chart of a method for determining precoder parametersfor a wireless device, according to an embodiment;

FIG. 7 is a block diagram of a wireless device configured to determineparameters of a precoder according to an embodiment:

FIG. 8 is a block diagram of a wireless device configured to determineparameters of a precoder according to another embodiment:

FIG. 9 is a block diagram of a network node, such as an eNodeB,configured to determine transmission parameters for a wireless device,according to an embodiment;

FIG. 10 is a flow chart of a method for sending precoder parameters in awireless communication system, according to an embodiment;

FIG. 11 is a flow chart of a method for determining precoder parametersin a wireless communication system, according to an embodiment;

FIG. 12 is flow chart for determining transmission parameters in awireless communication system, according to an embodiment:

FIG. 13 is flow chart for determining transmission parameters in awireless communication system, according to another embodiment:

FIG. 14 is a block diagram of a network node configured to determinetransmission parameters for a wireless device, according to anotherembodiment;

FIG. 15 is a block diagram of a wireless device configured to determineparameters of a precoder according to another embodiment;

FIG. 16 is a block diagram of a wireless device configured to determineparameters of a precoder according to another embodiment;

FIG. 17 is a block diagram of a wireless device configured to determineparameters of a precoder according to another embodiment; and

FIG. 18 is a block diagram of a network node configured to determinetransmission parameters for a wireless device, according to anotherembodiment.

DETAILED DESCRIPTION

Before describing in detail exemplary embodiments, it is noted that theembodiments reside in combinations of apparatus components andprocessing steps related to a factorized precoder structure formulti-beam precoder codebooks.

Accordingly, components have been represented where appropriate byconventional symbols in the drawings, showing only those specificdetails that are pertinent to understanding the embodiments so as not toobscure the disclosure with details that will be readily apparent tothose of ordinary skill in the art having the benefit of the descriptionherein.

As used herein, relational terms, such as “first” and “second,” “top”and “bottom,” and the like, may be used solely to distinguish one entityor element from another entity or element without necessarily requiringor implying any physical or logical relationship or order between suchentities or elements.

Embodiments of the present disclosure may be implemented in a wirelessnetwork such as the example wireless communication network/systemillustrated in FIG. 3. However, the embodiments may be implemented inany appropriate type of system using any suitable components.

FIG. 3 illustrates an example of a wireless communication network 300that may be used for wireless communications. Wireless communicationnetwork 300 includes wireless devices 310 (e.g., user equipments, UEs)and a plurality of network nodes 320 (e.g., eNBs, gNBs, base stations,etc.) connected to one or more core network nodes 340 via aninterconnecting network 330. Wireless devices 310 within a coverage areamay each be capable of communicating directly with network nodes 320over a wireless interface. In certain embodiments, wireless devices 310may also be capable of communicating with each other viadevice-to-device (D2D) communication. In certain embodiments, networknodes 320 may also be capable of communicating with each other, e.g. viaan interface (e.g. X2 in LTE or other suitable interface).

As an example, wireless device 310 may communicate with network node 320over a wireless interface. That is, wireless device 310 may transmitwireless signals and/or receive wireless signals from network node 320.The wireless signals may contain voice traffic, data traffic, controlsignals, and/or any other suitable information. In some embodiments, anarea of wireless signal coverage associated with a network node 320 maybe referred to as a cell.

In some embodiments, wireless device 310 may be interchangeably referredto by the non-limiting term user equipment (UE). It refers to any typeof wireless device communicating with a network node and/or with anotherUE in a cellular or mobile communication system. Examples of UE aretarget device, device to device (D2D) UE, machine type UE or UE capableof machine to machine (M2M) communication, Personal Digital Assistant(PDA), tablet computer, mobile terminals, smart phone, laptop embeddedequipped (LEE), laptop mounted equipment (LME), Universal Serial Bus(USB) dongles, narrowband Internet of Things (NB-IoT) UE, etc. Exampleembodiments of a wireless device 310 are described in more detail belowwith respect to FIGS. 15-17.

The “network node” can correspond to any type of radio network node orany network node, which communicates with a UE and/or with anothernetwork node. Examples of network nodes are Base stations, e.g., a RadioBase Station (RBS), which may be sometimes referred to herein as, e.g.,evolved NodeB “eNB”, “eNodeB”, “NodeB”, “B node”, “gNB” or BTS (BaseTransceiver Station), depending on the technology and terminology used.The base stations may be of different classes such as, e.g., macroeNodeB, home eNodeB or pico base station, based on transmission powerand thereby also cell size. A cell is the geographical area where radiocoverage is provided by the base station at a base station site. Onebase station, situated on the base station site, may serve one orseveral cells. Further, each base station may support one or severalcommunication technologies. The base stations communicate over the airinterface operating on radio frequencies with the terminals within rangeof the base stations. In the context of this disclosure, the expressionDownlink (DL) is used for the transmission path from the base station tothe mobile station. The expression Uplink (UL) is used for thetransmission path in the opposite direction i.e., from the mobilestation to the base station.

In certain embodiments, network nodes 320 may interface with a radionetwork controller (not shown). The radio network controller may controlnetwork nodes 320 and may provide certain radio resource managementfunctions, mobility management functions, and/or other suitablefunctions. In certain embodiments, the functions of the radio networkcontroller may be included in the network node 320. The radio networkcontroller may interface with the core network node 340. In certainembodiments, the radio network controller may interface with the corenetwork node 340 via the interconnecting network 330.

The interconnecting network 330 may refer to any interconnecting systemcapable of transmitting audio, video, signals, data, messages, or anycombination of the preceding. The interconnecting network 330 mayinclude all or a portion of a public switched telephone network (PSTN),a public or private data network, a local area network (LAN), ametropolitan area network (MAN), a wide area network (WAN), a local,regional, or global communication or computer network such as theInternet, a wireline or wireless network, an enterprise intranet, or anyother suitable communication link, including combinations thereof.

In some embodiments, the core network node 340 may manage theestablishment of communication sessions and various otherfunctionalities for wireless devices 310. In certain embodiments,network nodes 320 may interface with one or more other network nodesover an internode interface. For example, network nodes 320 mayinterface each other over an X2 interface.

Although FIG. 3 illustrates a particular arrangement of network 300, thepresent disclosure contemplates that the various embodiments describedherein may be applied to a variety of networks having any suitableconfiguration. For example, network 300 may include any suitable numberof wireless devices 310 and network nodes 320, as well as any additionalelements suitable to support communication between wireless devices orbetween a wireless device and another communication device (such as alandline telephone). The embodiments may be implemented in anyappropriate type of telecommunication system supporting any suitablecommunication standards and using any suitable components, and areapplicable to any radio access technology (RAT) or multi-RAT systems inwhich the wireless device receives and/or transmits signals (e.g.,data). While certain embodiments are described for New Radio (NR) and/orLTE, the embodiments may be applicable to any RAT, such as UTRA, E-UTRA,narrow band internet of things (NB-IoT), WiFi, Bluetooth, nextgeneration RAT (NR, NX), 4G, 5G, LTE Frequency Division Duplex(FDD)/Time Division Duplex (TDD), etc.

It should be noted that functions described herein as being performed bya base station may be distributed over a plurality of base stationsand/or network nodes. Further, although embodiments are described withreference to base stations, it is understood that embodiments can beimplemented in or across any suitable network node, of which basestations are a type. Also, the network 300 may allow for Multi-UserMultiple Input Multiple Output (MU-MIMO) transmission. As such, network300 may be referred to as a MU-MIMO wireless communication network orsystem.

Embodiments provide a precoder structure for multi-beam precoderfeedback that utilizes various properties to keep down the feedbackoverhead. Some embodiments provide increased MU-MIMO performance ascompared with known arrangements by having rich precoder feedback withreasonable feedback overhead. Codebooks having multi-beam precoders thathave specific structures, allowing for low feedback overhead aredisclosed.

Consider first the time-domain channel between a size-N co-polarizeduniform linear array (ULA) with d_(λ) antenna element separation inwavelengths and a single receive antenna. The channel matrix may beexpressed in the general form

${H(\tau)} = {{h^{T}(\tau)} = {\sum\limits_{i = 1}^{M}\; {c_{i}{\alpha^{T}\left( \theta_{i} \right)}{\delta \left( {\tau - \tau_{i}} \right)}}}}$

i.e., consisting of a sum of M multi-path components, where c_(i) is acomplex channel coefficient,

${\alpha (\theta)} = \begin{bmatrix}1 \\e^{j\; 2\; {\pi \cdot 1 \cdot d_{\lambda}}{\cos {(\theta)}}} \\\vdots \\e^{j\; 2\; {\pi \cdot {({N - 1})} \cdot d_{\lambda}}{\cos {(\theta)}}}\end{bmatrix}$

is an array steering vector, θ_(i) is an angle of departure (AoD)relative to the ULA of multi-path component i and τ_(i) is itspropagation delay.

The frequency-domain representation of the channel matrix is thenderived as

${H(f)} = {{h^{T}(f)} = {{\int_{\tau = {- \infty}}^{\infty}{\sum\limits_{i = 1}^{M}\; {c_{i}{\alpha^{T}\left( \theta_{i} \right)}{\delta \left( {\tau - \tau_{i}} \right)}e^{{- j}\; 2\; \pi \; f\; \tau}d\; \tau}}}\  = {{\sum\limits_{i = 1}^{M}\; {c_{i}{\alpha^{T}\left( \theta_{i} \right)}{\int_{\tau = {- \infty}}^{\infty}{\delta \left( {\tau - \tau_{i}} \right)e^{{- j}\; 2\; \pi \; f\; \tau}d\; \tau}}}} = {\sum\limits_{i = 1}^{M}\; {c_{i}{\alpha^{T}\left( \theta_{i} \right)}e^{{- j}\; 2\; \pi \; f\; \tau_{i}}}}}}}$

Consider now the channel matrix for a certain frequency f=f₀. Thechannel vector then becomes h^(T)(f)=Σ_(i=1)^(M)c_(i)a^(T)(θ_(i))e^(−j2πf) ⁰ ^(τ) ^(i) =Σ_(i=1) ^(M){tilde over(c)}_(i)a^(T)(θ_(i)), where {tilde over (c)}_(i) is another complexcoefficient. The optimal precoder that perfectly inverts this channel isthe maximum ratio transmission (MRT) precoder w_(MRT)=(h^(T)(f))^(H)=h⁺,wherein * denotes the complex conjugate.

D_(N) is defined as a size N×N DFT matrix, i.e., the elements of D_(N)are defined as

$\left\lbrack D_{N} \right\rbrack_{k,l} = {\frac{1}{\sqrt{N}}{e^{\frac{{j\; 2\; \pi \; k\; l}\;}{N}}.}}$

Further,

${R_{N}(q)} = {{diag}\left( \begin{bmatrix}e^{j\; 2\; {\pi \cdot 0 \cdot \frac{q}{N}}} & e^{j\; 2\; {\pi \cdot 1 \cdot \frac{q}{N}}} & \ldots & e^{j\; 2\; {\pi \cdot {({N - 1})} \cdot \frac{q}{N}}}\end{bmatrix} \right)}$

to be a size N×N rotation matrix, defined for 0≤q<1. Multiplying D_(N)with R_(N)(q) from the left creates a rotated DFT matrix with entries

$\left\lbrack {{R_{N}(q)}D_{N}} \right\rbrack_{k,l} = {\frac{1}{\sqrt{N}}{e^{\frac{{j\; 2\; \pi \; k\; {({l + q})}}\;}{N}}.}}$

The rotated DFT matrix R_(N)(q)D_(N)=[d₁ d₂ . . . d_(N)] consists ofnormalized orthogonal column vectors {d_(i)}_(i=1) ^(N) whichfurthermore span the vector space

^(N). That is, the columns of R_(N)(q)D_(N), for any q, is anorthonormal basis of

^(N).

The MRT precoder is multiplied with the rotated DFT matrix in order todo a basis change from, so called, antenna space to beam space. Theresulting beam space representation of the precoder vector may then beexpressed as w_(B) ^(T)=w_(MRT)^(H)R_(N)(q)D_(N)=h^(T)R_(N)(q)D_(N)=h^(T)[d₁ d₂ . . . d_(N)]=[Σ_(i=1)^(M){tilde over (c)}_(i)a^(T)(θ_(i))d₁ Σ_(i=1) ^(M){tilde over(c)}_(i)a^(T)(θ_(i))d₂ . . . Σ_(i=1) ^(M){tilde over(c)}_(i)a^(T)(θ_(i))d_(N)].

Note first that the steering vector

${\alpha (\theta)} = \begin{bmatrix}1 \\e^{j\; 2\; {\pi \cdot 1 \cdot d_{\lambda}}{\cos {(\theta)}}} \\\vdots \\e^{j\; 2\; {\pi \cdot {({N - 1})} \cdot d_{\lambda}}{\cos {(\theta)}}}\end{bmatrix}$

may be expressed as scaled column of a rotated DFT matrix[R_(N)(q)D_(N)]_(:,l) with l=└d_(λ) cos(θ)┘ and q=d_(λ) cos(θ)−└d_(λ)cos(θ)┘. Note that a conjugated steering vector a*(θ) is equal toanother steering vector with the angle mirrored at the broadside of thearray, i.e., a*(θ)=a(π−θ).

Now moving back to the beam space representation of the precoder vectorw_(B) ^(T), note that a^(T)(θ_(i))d_(l) is the inner product between aconjugated steering vector and a column of a rotated DFT matrix. It waspreviously noted that any steering vector could be expressed as a scaledcolumn of a rotated DFT matrix (with appropriate values set for q=q₀ andl=l₀). In that case, the inner product between the (conjugated) steeringvector and d_(l) will be

${{\alpha^{T}\left( \theta_{i} \right)}d_{l}} = \left\{ {\begin{matrix}{\sqrt{N},} & {l = l_{0}} \\{0,} & {l \neq l_{0}}\end{matrix}.} \right.$

Again, this requires that q is set appropriately so that the beam spaceis rotated to fit the steering vector of multi-path coefficient iperfectly. If that is not the case, the steering vector will still besparse in the beam space coordinate system, with one or two coefficientshaving a large magnitude and the rest of the coefficients having a lowmagnitude. Each multi-path component will thus, to a large extent, onlycontribute to one or a few beam space coefficients. The impact of beamspace rotation on the sparseness of the beam space channel isillustrated in FIGS. 4A-4D, in which a Line-of-Sight (LoS) channel isshown. FIG. 4A is for a rotation index/factor of q=0. FIG. 4B is for arotation index/factor of q=2/4. FIG. 4C is for a rotation index/factorof q=1/4. FIG. 4D is for a rotation index/factor of q=3/4.

However, the frequency-domain channel is a sum of M multi-pathcomponents each with a possibly different angle of departure θ_(i). Thebeam space sparseness of the channel is thus dependent on thedistribution of the multi-path components AoD θ_(i). The spread in thisdistribution is often denoted as the angular spread of a channel. A pureLine-of-Sight (LoS) channel has low angular spread and can be verysparsely represented in beam space, as is illustrated in FIGS. 4A-4D. Achannel with very large angular spread, on the other hand, cannot besparsely represented in beam space, but will need to be represented bymany beam space coefficients. However, a cellular wireless channeltypically has only a few strong enough multi-path components, and canthus be effectively represented with only a few beam space coefficients.This is what is exploited by the multi-beam codebooks presented herein.

To elucidate the precoder structure of some embodiments, the (rotated)DFT matrices that were appropriate transforms for a single-polarized ULAare extended to also fit the more general case of dual-polarized 2Duniform planar arrays (UPAs).

A rotated 2D DFT matrix is defined as D_(N) _(V) _(,N) _(H)(q_(V),q_(H))=(R_(N) _(H) (q_(H))D_(N) _(H) )⊗(R_(N) _(V) (q_(V))D_(N)_(V) )=[d₁ d₂ . . . d_(N) _(V) _(N) _(H) ]. The columns {d_(i)}_(i=1)^(N) ^(DP) of D_(N) _(V) _(,N) _(H) (q_(V),q_(H)) constitutes anorthonormal basis of the vector space

^(N) ^(V) ^(N) ^(H) . Such a column d_(i) is henceforth denoted a (DFT)beam.

Consider now a dual-polarized UPA, where the channel matrix H=[H_(pol1)H_(pol2)].

Create a dual-polarized beam space transformation matrix

${B_{N_{V},N_{H}}\left( {q_{V},q_{H}} \right)} = {{I_{2} \otimes {D_{N_{V},N_{H}}\left( {q_{V},q_{H}} \right)}} = {\quad{\begin{bmatrix}{D_{N_{V},N_{H}}\left( {q_{V},q_{H}} \right)} & 0 \\0 & {D_{N_{V},N_{H}}\left( {q_{V},q_{H}} \right)}\end{bmatrix} = {\quad{\left\lbrack \begin{matrix}d_{1} & d_{2} & \ldots & d_{N_{V}N_{H}} & 0 & 0 & \ldots & 0 \\0 & 0 & \ldots & 0 & d_{1} & d_{2} & \ldots & d_{N_{V}N_{H}}\end{matrix} \right\rbrack = {\quad{\begin{bmatrix}b_{1} & b_{2} & \ldots & b_{2\; N_{V}N_{H}}\end{bmatrix}.}}}}}}}$

The columns {b_(i)}_(i=1) ^(2N) ^(V) ^(N) ^(H) of B_(N) _(V) _(,N) _(H)(q_(V),q_(H)) constitutes an orthonormal basis of the vector space

^(2N) ^(V) ^(N) ^(H) . Such a column b_(i) is henceforth denoted asingle-polarized beam (SP-beam) as it is constructed by a beam dtransmitted on a single polarization

$\left( {{i.e.},{b = {{\begin{bmatrix}d \\0\end{bmatrix}\mspace{14mu} {or}\mspace{14mu} b} = \begin{bmatrix}0 \\d\end{bmatrix}}}} \right).$

Also introduced is the notation “dual-polarized beam” to refer to a beamtransmitted on both polarizations (co-phased with an (arbitrary)co-phasing factor e^(jα), i.e.,

$\left. {b_{DP} = \begin{bmatrix}{d.} \\{e^{j\; \alpha}{d.}}\end{bmatrix}} \right).$

It should be noted that the co-phasing factors can be used to make thetransmitted beams from the two polarizations within a layer (of amulti-layer transmission) add up coherently (i.e. in-phase) at thereceiver in order to increase the received power of that layer, which inturn increases the received SINR of that layer. The co-phasing factorscan also make the different layers (in case of a rank-2 transmission orhigher) be received orthogonal towards one another in order to minimizeinter-layer interference, which also leads to increase the received SINRof the layers.

Utilizing the assumption that the channel is somewhat sparse, much ofthe channel energy can be sufficiently captured by only selecting acolumn subset of B_(N) _(V) _(,N) _(H) (q_(V),q_(H)). That is, it issufficient to describe a couple of the SP-beams, which keeps down thefeedback overhead. A column subset Is consisting of N_(SP) columns ofB_(N) _(V) _(,N) _(H) (q_(V),q_(H)) is selected to create a reduced beamspace transformation matrix B_(I) _(S) =[b₁ _(S) ₍₁₎ b₁ _(S) ₍₂₎ . . .b₁ _(S) _((N) _(SP)) ]. In other words, select columns number I_(S)=[1 510 25] to create the reduced beam space transformation matrix B_(I) _(S)=[b₁ b₅ b₁₀ b₂₅], as one non-limiting example.

Furthermore, it should be noted that a precoder matrix w may be derivedfrom eigenvalues of the channel matrix H. More specifically, theprecoder w may be calculated to be approximately equal to the principaleigenvectors of the channel matrix H. For example, in the case of asingle receive antenna, which can thus support only a single layertransmission, the strongest eigenvector (v1) is equal to the MRTprecoder, i.e. w_(MRT)=h*=v1.

A general precoder structure for precoding a single layer is as follows:

$w = {{B_{I_{S}}\begin{bmatrix}c_{1} \\c_{2} \\\vdots \\c_{N_{SP}}\end{bmatrix}} = {{\begin{bmatrix}b_{I_{S}{(1)}} & b_{I_{S}{(2)}} & \ldots & b_{I_{S}{(N_{SP})}}\end{bmatrix}\begin{bmatrix}c_{1} \\c_{2} \\\vdots \\c_{N_{SP}}\end{bmatrix}} = {\sum\limits_{i = 1}^{N_{SP}}{c_{i}{b_{I_{S}{(i)}}.}}}}}$

where {c_(i)}_(i=1) ^(N) ^(SP) are complex coefficients. A more refinedmulti-beam precoder structure is achieved by separating the complexcoefficients in a power (or amplitude) and a phase part as

$\begin{matrix}{w = {{B_{I_{S}}\begin{bmatrix}c_{1} \\c_{2} \\\vdots \\c_{N_{SP}}\end{bmatrix}} = {{B_{I_{S}}\begin{bmatrix}{\sqrt{p_{1}}e^{j\; \alpha_{1}}} \\{\sqrt{p_{2}}e^{j\; \alpha_{2}}} \\\vdots \\{\sqrt{p_{N_{SP}}}e^{j\; \alpha_{N_{SP}}}}\end{bmatrix}} = {{{B_{I_{S}}\begin{bmatrix}\sqrt{p_{1}} & \; & 0 & \; & \; & \; \\\; & \; & \; & \; & \ddots & \; \\0 & \; & \sqrt{p_{2}} & \; & \; & \; \\\; & \; & \; & \ddots & \; & 0 \\\; & \ddots & \; & \; & \; & \; \\\; & \; & \; & 0 & \; & \sqrt{p_{N_{SP}}}\end{bmatrix}}\begin{bmatrix}e^{j\; \alpha_{1}} \\e^{j\; \alpha_{2}} \\\vdots \\e^{j\; \alpha_{N_{SP}}}\end{bmatrix}} = {B_{I_{S}}{\sqrt{P}\begin{bmatrix}e^{j\; \alpha_{1}} \\e^{j\; \alpha_{2}} \\\vdots \\e^{j\; \alpha_{N_{SP}}}\end{bmatrix}}}}}}} & {{equation}\mspace{14mu} 4}\end{matrix}$

As multiplying the precoder vector w with a complex constant C does notchange its beamforming properties (as only the phase and amplituderelative to the other single-polarized beams is of importance), one maywithout loss of generality assume that the coefficients corresponding,to e.g., SP-beam1, is fixed to p₁=1 and e^(jα) ¹ =1, so that parametersfor one less beam may be signaled from the wireless device to the basestation. Furthermore, the precoder may be further assumed to bemultiplied with a normalization factor, so that a sum power constraintis fulfilled, i.e., that ∥w∥²=1. Any such normalization factor isomitted from the equations herein for clarity.

Once the wireless device has determined the precoder matrix, thefollowing information should be fed back by the wireless device to thebase station, e.g., eNodeB, in a CSI feedback report, for example:

the chosen columns of B_(N) _(V) _(,N) _(H) (q_(V),q_(H)), i.e., theN_(SP) single-polarized beams. This requires at most N_(SP)·log₂2N_(V)N_(H) bits;

The vertical and horizontal DFT basis rotation factors q_(V) and q_(H).For instance, the

${{q(i)} = \frac{i}{Q}},$

(i)=i=0, 1, . . . , Q−1, for some value of Q. The corresponding overheadwould then be 2 log₂ Q bits:

The (relative) power levels {p₂, p₃, . . . , p_(N) _(SP) } of theSP-beams. If L is the number of possible discrete power levels,(N_(SP)−1)·log₂ L is needed to feed back the SP-beam power levels; and

The co-phasing factors {e^(jα) ² , e^(jα) ³ , . . . ,

e^(j α_(N_(SP)))}

of the SP-beams. For instance,

${{\alpha (k)} = \frac{2\pi \; k}{K}},$

k=0, 1, . . . K−1, for some value of K. The corresponding overhead wouldbe (N_(SP)−1)·log₂ K.

In the following examples, further optimizations can be performed inorder to decrease the CSI feedback overhead.

In some embodiments, the possible choices of columns of B_(N) _(V) _(,N)_(H) (q_(V),q_(H)) are restricted so that if column i=i₀ is chosen, sois column i=i₀+N_(V)N_(H). That is, if an SP-beam corresponding to acertain beam mapped to the first polarization is chosen, e.g.,

${b_{i_{0}} = \begin{bmatrix}d_{i_{0}} \\0\end{bmatrix}},$

this would imply that the SP-beam

${b_{i_{0}} + {N_{V}N_{H}}} = \begin{bmatrix}0 \\d_{i_{0}}\end{bmatrix}$

is chosen as well. That is, the SP-beam corresponding to the saidcertain beam mapped to the second polarization is chosen as well. Thiswould reduce the feedback overhead as only N_(DP)=N_(SP)/2 columns ofB_(N) _(V) _(,N) _(H) (q_(V),q_(H)) would have to be selected andsignaled back to the base station. In other words, the column selectionis done on a beam (or DP-beam) level rather than an SP-beam level. If acertain beam is strong on one of the polarizations it would typicallyimply that the beam would be strong on the other polarization as well,at least in a wideband sense, so the loss of restricting the columnselection in this way would not significantly decrease the performance.

In one embodiment, the beams are sorted in power strength. Thequantization of relative powers may then be coarser for beams with weakpower to save feedback bits. In another embodiment, only the index ofthe strongest beam is pointed out, the other beams are given in an orderthat does not depend on the power strength. Specifying the beams in anunordered fashion may save feedback bits.

In some embodiments, the multi-beam precoder is factorized into two ormore factors that are selected with different frequency-granularity, inorder to reduce the feedback overhead. In a preferred such embodiment,the SP-beam selection (i.e., the choice of matrix B_(I) _(S) ) and therelative SP-beam powers/amplitudes (i.e., the choice of matrix √{squareroot over (P)}) is selected with a certain frequency-granularity whilethe SP-beam phases (i.e., the choice of matrix

$\left. \quad\begin{bmatrix}e^{j\; \alpha_{1}} \\e^{j\; \alpha_{2}} \\\vdots \\e^{j\; \alpha_{N_{SP}}}\end{bmatrix} \right)$

is selected with another certain frequency-granularity. In one suchembodiment, the said certain frequency-granularity corresponds to awideband selection (that is, one selection for the entire bandwidth ofthe carrier) while the said another certain frequency-granularitycorresponds to a per-subband selection (that is, the carrier bandwidthis split into a number of subbands, typically consisting of 1-10physical resource blocks (PRBs), and a separate selection is done foreach subband).

In a typical such embodiment, the multi-beam precoder vector isfactorized as w=W₁W₂, where W₁ is a first factor and can be selected (orgenerated) with a certain frequency-granularity and W₂ is a secondfactor and can be selected (or generated) with another certainfrequency-granularity. The precoder vector may then be expressed as

$w = {{\underset{\underset{= W_{1}}{}}{B_{I_{S}}\sqrt{P}}\underset{\underset{= W_{2}}{}}{\begin{bmatrix}e^{j\; \alpha_{1}} \\e^{j\; \alpha_{2}} \\\vdots \\e^{j\; \alpha_{N_{SP}}}\end{bmatrix}}} = {W_{1}{W_{2}.}}}$

Using this notation, if the said certain frequency-granularitycorresponds to a wideband selection of W₁ and the said another certainfrequency-granularity corresponds to a per-subband selection of W₂, theprecoder vector for subband l may be expressed as w_(l)=W₁W₂(l). Thatis, only W₂ is a function of the subband index l.

In a more general version of the previous sets of embodiments, thecriterion that the multi-beam precoder vector w is composed of two ormore matrix factors are dropped. Instead, the choice of w may beexpressed as a selection of two or more precoder indices, i.e., i₁, i₂,. . . , where the precoder indices may be selected with differentfrequency-granularity. That is, the precoder vector may be expressed asa function of the two or more precoder indices i₁, i₂, . . . , so thatw(i₁, i₂, . . . ). In a preferred embodiment, i₁ may be selected on awideband basis while i₂ may be selected on a per-subband basis so thatthe precoder vector for subband l may be expressed as w_(l)=w(i₁,i₂(l)).

The previous embodiments have been presented assuming a precoder vectorw for single-layer transmission (i.e., transmission rank one) but areapplicable for multi-layer transmission (i.e., transmission rank largerthan one) using a precoder matrix W as well. The following embodimentsfurther concern precoder matrix designs for multi-layer transmission.

In some embodiments, the precoder matrix is constructed by keeping theSP-beam selection and the relative SP-beam powers/amplitudes the samefor all layers of the multi-layer transmission and only changing theSP-beam phases for the different layers. That is, the multi-beamprecoder matrix for multi-layer transmission may be expressed as

${W = {B_{I_{S}}{\sqrt{P}\begin{bmatrix}e^{j\; \alpha_{1,1}} & e^{j\; \alpha_{1,2}} & \ldots & e^{j\; \alpha_{1,R}} \\e^{j\; \alpha_{2,1}} & e^{j\; \alpha_{2,2}} & \ldots & e^{j\; \alpha_{2,R}} \\\; & \vdots & \; & \; \\e^{j\; \alpha_{N_{SP},1}} & e^{j\; \alpha_{N_{SP},2}} & \ldots & e^{j\; \alpha_{N_{SP},R}}\end{bmatrix}}}},$

where R is the number of layers in the multi-layer transmission, i.e.,the transmission rank.

In another embodiment, some of the entries in the phase matrix

$\quad\begin{bmatrix}e^{j\; \alpha_{1,1}} & e^{j\; \alpha_{1,2}} & \ldots & e^{j\; \alpha_{1,R}} \\e^{j\; \alpha_{2,1}} & e^{j\; \alpha_{2,2}} & \ldots & e^{j\; \alpha_{2,R}} \\\; & \vdots & \; & \; \\e^{j\; \alpha_{N_{SP},1}} & e^{j\; \alpha_{N_{SP},2}} & \ldots & e^{j\; \alpha_{N_{SP},R}}\end{bmatrix}$

are allowed to be zero, so as to not use all of the selected SP-beams totransmit all of the layers.

Note that the previous embodiments regarding multi-layer transmissionmay be combined with the embodiments regarding differentfrequency-granularities of precoder factors. For instance, W₁=B_(I) _(S)√{square root over (P)} and

$W_{2} = \begin{bmatrix}e^{j\; \alpha_{1,1}} & e^{j\; \alpha_{1,2}} & \ldots & e^{j\; \alpha_{1,R}} \\e^{j\; \alpha_{2,1}} & e^{j\; \alpha_{2,2}} & \ldots & e^{j\; \alpha_{2,R}} \\\; & \vdots & \; & \; \\e^{j\; \alpha_{N_{SP},1}} & e^{j\; \alpha_{N_{SP},2}} & \ldots & e^{j\; \alpha_{N_{SP},R}}\end{bmatrix}$

so that the first matrix factor (or first factor) W₁ is common betweenall layers of the multi-layer transmission while the second matrixfactor (or second factor) W₂ contains the layer-specific precoding.

In some embodiments, the precoder structure for a rank-2 precoder isconsidered and it is assumed that the selection of columns of B_(N) _(V)_(,N) _(H) (q_(V),q_(H)) is done on a DP-beam basis rather than anSP-beam basis, as disclosed in an earlier embodiment. In theseembodiments, the phase selection for the precoder for the second layeris a function of the phase selection for the precoder for the firstlayer. In one such embodiment, the phases for the second layer thatcorresponds to the first polarization is equal to the phases for thefirst layer that corresponds to the first polarization, while the phasesfor the second layer that corresponds to the second polarization is thenegation of the phases for the first layer that corresponds to thesecond polarization. A negation corresponds to a phase shift of 180degrees. Constructing the precoder in this fashion ensures that the twolayers are orthogonal.

The following embodiments concern how the relative power levels {p₂, p₃,. . . p_(N) _(SP) } of the SP-beams are quantized. It can be noted thatthe relative power levels may be larger than zero and smaller than one,since one may assume that the first selected SP-beam corresponds to thestrongest SP-beam. In one embodiment, the beam powers are uniformlyquantized between [p_(min), 1], where p_(min) corresponds to a minimumpower level (which may be equal to zero). In one embodiment, a monotonicfunction of the beam powers is uniformly quantized. In one suchembodiment, the square root of the beams power (i.e., √{square root over(p_(i))}) is uniformly quantized.

In another such embodiment, the quantization is done in the dB-domain sothat the values 10 log₁₀ p_(i) are uniformly quantized in the interval[p_(min,dB), 0] dB instead. Note here that p_(min,dB)<0.

The following embodiments concern how such feedback as described hereinmay be calculated by the wireless device, i.e., they are wireless deviceimplementation embodiments.

In these embodiments, the wireless device selects the phases

{e^(j α₂), e^(j α₃), …  , e^(j α_(N_(SP)))}

of the SP-beams, where each phase may be selected from a set of possiblevalues, for instance,

${{\alpha (k)} = \frac{2\pi \; k}{K}},$

k=0, 1, . . . K−1, for some value of K. There are thus K^(N) ^(SP) ⁻¹possible combinations, which may be very large if K or N_(SP) is largeand it may thus be infeasible for the wireless device to do anexhaustive search of all possibilities. Instead, the wireless device mayperform sequential co-phasing. That is, the wireless device firstsearches through the K possibilities for the first co-phasing factore^(jα2) (by e.g., calculating the received power of the precoderhypothesis) while setting all remaining N_(SP)−2 co-phasing factors tozero. It then searches through the K possibilities for the secondco-phasing factor e^(jα) ³ while setting the remaining N_(SP)−3co-phasing factors to zero, and so forth. Instead of searching throughall the K^(N) ^(SP) ⁻¹ possible combination, the wireless device onlyhas to search through K (N_(SP)−1) hypotheses.

The wireless device may also select which SP-beams should be included inthe precoder, i.e., how to select columns from the dual-polarized beamspace transformation matrix B_(N) _(V) _(,N) _(H) to form the reducedbeam space transformation matrix B_(I) _(S) . First, the wireless devicemay form an averaged channel correlation matrix by averaging infrequency corresponding to the frequency-granularity of the beamselection (e.g., over the entire bandwidth) as R=Σ_(f) H^(H)H. Then, itmay calculate the wideband received power of each SP-beam by taking thediagonal elements of the matrix product B_(N) _(V) _(,N) _(H) ^(H)RB_(N)_(V) _(,N) _(H) . The wireless device may then select the N_(SP) beamswhich have the largest wideband received power. The received power of a(hypothetical) beam i is given by: ∥Hb_(i)∥². The relative power levelsp of the (hypothetical) beams in the precoder can be set to correspondto the relative received powers of the beams,

${{i.e.\text{:}}\mspace{14mu} \frac{p_{1}}{p_{2}}} = {\frac{{{Hb}_{1}}^{2}}{{{Hb}_{2}}^{2}}.}$

One embodiment concerns how the rotation factors q_(V) and q_(H) may becalculated by the wireless device. It is assumed that the rotationfactors may be selected from a fixed set of possible values, forinstance,

${{q(i)} = \frac{i}{Q}},$

i=0, 1, . . . , Q−1, for some value of Q. The wireless device may then,for each possible value of the rotation factors (q_(V), q_(H)),calculate the received power of the N_(SP) strongest beams correspondingto the rotated beam space transformation matrix B_(N) _(V) _(,N) _(H)(q_(V),q_(H)) according to the previous “beam selection method”embodiment. The wireless device may then select the rotation hypothesisthat maximizes the received power in the reduced beam space.

It should be noted that in the selection of beams, the beams are notbeams transmitted by the network node, but they are hypothesizedtransmissions that the wireless device evaluates. The network node/basestation transmits a set of non-precoded CSI-RS (from e.g. each antennaelement of the antenna array) which is measured by the wireless device,which can then be used to determine a channel estimate H. Based on thischannel estimate, the wireless device will select an optimal precoder(which is comprised by a sum of orthogonal DFT beams). For example, toselect the best beams, the wireless device will perform a search overthe differently rotated orthogonal DFT bases/matrix B_(N) _(V) _(,N)_(H) (q_(V),q_(H)) to:

1) select the best rotated orthogonal DFT basis/matrix B_(N) _(V) _(,N)_(H) (q_(V),q_(H)) and the corresponding rotation factors q_(V), q_(H);and

2) select the best N_(SP) beams from the basis/matrix B_(N) _(V) _(,N)_(H) (q_(V),q_(H)).

FIG. 5 illustrates a signaling diagram 500 between a network node 320,such as an eNB, and the wireless device 310, in a wireless communicationnetwork/system 300, for example, for reporting CSI feedback from thewireless device to the eNB.

The network node 320 first sends reference signals to the wirelessdevice 310, such as the CSI-RS or CRS, or any other signals that allowto determine or provide information regarding the channel (step 510).

Based on the received reference signals, the wireless device 310determines the parameters of a precoder (step 520). For example, thewireless device can determine an optimal precoder for the channelconditions/estimate based on the received reference signals.

Once the precoder parameters are determined, the wireless device 310sends a CSI report to the network node, the CSI report including thedetermined precoder parameters (step 530).

Once the network node 320 receives the CSI report, it determinestransmission parameters based on the received information (e.g.parameters of the precoder). For example, the network node 320 candecide to use the precoder recommended by the wireless device todetermine a Modulation and Coding scheme (MCS) and use the precodingscheme of the precoder for the wireless device's data transmission.However, based on the received information, the network node 320 maydecide to use another precoder and determine the MCS and precodingscheme based on this precoder (step 540).

It should be noted that the signaling diagram 500 is known in the art.Embodiments of the present disclosure are directed to how the wirelessdevice 310 determines the parameters of a precoder to recommend to thenetwork node 320. As an example, the wireless device 310 can select somebeams, which have the largest received power, for example, from aplurality of orthogonal beams. To calculate the power level of thebeams, the wireless device takes the diagonal elements of the matrixproduct B_(N) _(V) _(,N) _(H) ^(H)RB_(N) _(V) _(,N) _(H) , whereR=Σ_(f)H^(H)H as described above. The wireless device also determinesthe phases of the selected beams. To determine the phases, the wirelessdevice 310 may use the sequential co-phasing method, as described above.The wireless device may also calculate the rotation factors q_(V) andq_(H) which are used to obtain the orthogonal beams (d) and calculatethe beam space transformation matrix B_(N) _(V) _(,N) _(H) . Othermethods for determining the power levels could be also used. Forexample, the wireless device could potentially do a full exhaustivesearch over all precoder hypotheses and calculate an estimate of thethroughput achievable with each precoder.

Once the parameters of the precoder are determined, the wireless devicesends the CSI report to the base station, the CSI report including theparameters of the precoder. According to some embodiments, theparameters of the precoder include the indices corresponding to theselected beams, their power levels and phases, and the rotation factors.

In one embodiment, FIG. 6 is a flowchart of an exemplary processperformed at a wireless device 310 for determining parameters to enableconstruction of a precoder codebook structure in a wirelesscommunication system, according to an embodiment. The process includesselecting a subset of columns of a beam space transformation matrix,B_(N) _(V) _(,N) _(H) (q_(V),q_(H)), each column corresponding to asingle polarized beam (block 610). The process also includes factorizingeach column into at least two factors, a first factor having a firstfrequency granularity and a second factor having a second frequencygranularity (block 620).

FIG. 7 is a block diagram of an example wireless device 310 configuredto determine precoder information to enable construction of a precodercodebook structure in a wireless communication system, according to anembodiment.

The wireless device 310 has processing circuitry 700. In someembodiments, the processing circuitry 700 may include a memory 710 andprocessor 720, the memory 710 containing instructions which, whenexecuted by the processor 720, configure processor 720 to perform theone or more functions described herein, such as the steps of method 600.In addition to a traditional processor and memory, processing circuitry700 may comprise integrated circuitry for processing and/or control,e.g., one or more processors and/or processor cores and/or FPGAs (FieldProgrammable Gate Array) and/or ASICs (Application Specific IntegratedCircuitry).

The memory 710 is configured to store precoder information 730, theprecoder information including frequency granularities of factors ofsingle polarized beams. The processor is configured to select (740) asubset of columns of a beam space transformation matrix, each columncorresponding to an SP beam, the SP beam having phases. The processor720 also performs factorization (750) of each column into at least twofactors, wherein a first factor has a first frequency granularity and asecond factor has a second frequency granularity. The wireless device310 further includes a transmitter 760 configured to transmit thefactors and the frequency granularities to a base station.

FIG. 8 is a block diagram of an alternative embodiment of a wirelessdevice 310 configured to determine parameters to enable construction ofa precoder codebook structure in a wireless communication system,according to another embodiment. The wireless device 310 includes amemory module 800 (similar to 710 of FIG. 7) that stores precoderinformation 730. The wireless device also includes a beam selectormodule 810 configured to select a subset of columns of a beam spacetransformation matrix, each column corresponding to an SP beam, the SPbeam having phases. The wireless device also includes factorizationmodule 820 configured to factor each column into at least two factors,wherein a first factor has a first frequency granularity and a least asecond factor has a second frequency granularity. The wireless device310 further includes a transmitter module 830 configured to transmit thefactors and the frequency granularities to a base station.

FIG. 9 is a block diagram of a base station 320, such as an eNodeB ornetwork node, configured to transmit to a wireless device according totransmission parameters based on information received from the wirelessdevice, according to an embodiment. The base station 320 has processingcircuitry 900 having a memory 910 and a processor 920. The memory 910 isconfigured to store precoder information 930, contained in the CSIreport received from the wireless device. The processor 920 isconfigured to determine transmission parameters 940 including a rankindicator, modulation and coding scheme. The base station 320 has areceiver 950 configured to receive, from the wireless device, precoderinformation including: a subset of columns of a beam spacetransformation matrix, each column corresponding to a signal polarized,SP, beam, the SP beams having phases and amplitudes, and frequencygranularities of factors of the SP beams. The base station furthercomprises a transmitter 960 configured to transmit the transmissionparameters to the wireless device.

FIG. 10 illustrates a flow chart of a method 1000 for sending parametersof a precoder by a wireless device to a network node, in a wirelesscommunication system, such as 300.

The method starts with sending, to the network node, a subset of beamsselected from a plurality of orthogonal beams and power levels of theselected subset of beams, for a first frequency granularity (block1010).

The method then continues with sending, to the network node, phases ofthe selected subset of beams, for a second frequency granularity (block1020), wherein the selected subset of beams, the power levels and thephases of the selected subset of beams are part of the parameters of theprecoder. It should be noted that the parameters of the precoder sent tothe network node 320 may also comprise the rotation factors and otherinformation.

For example, the subset of beams is selected as explained above, tocreate the reduced space beam transformation matrix B_(I) _(S) . Theplurality of orthogonal beams corresponds to the columns of B_(N) _(V)_(,N) _(H) (q_(V),q_(H)), as an example. The first frequency granularitycorresponds to the (entire) frequency bandwidth (of a carrier) and thesecond frequency granularity corresponds to a frequency subband withinthe frequency bandwidth. Also, the power levels are the same for alllayers of a multi-layer transmission and the phases are specific to eachindividual layer of the multi-layer transmission, e.g. the phases areassignable independently for each individual layer.

It should be noted that the selected subset of beams, the power levelsand the phases can be sent to the network node in one message or inseveral messages, as will be appreciated by a person skilled in the art.

FIG. 11 illustrates a flow chart of a method 1100 for determiningparameters of a precoder in a wireless communication system, such as aMulti-User Multiple Input Multiple output (MU-MIMO) communicationsystem, according to another embodiment. The method is performed by awireless device, such as 310, for example. It should be noted thatmethod 1100 is similar to method 600 of FIG. 6, with the factoring step620 described in a different way.

Method 1100 starts with block 1110 by selecting a subset of beams from aplurality of orthogonal beams. The plurality of orthogonal beams cancorrespond to the columns of a rotated 2D DFT matrix, such as B_(N) _(V)_(,N) _(H) (q_(V),q_(H)). The selected subset of beams can correspond tothe columns of B_(I) _(S) , for example.

Method 1100 continues with obtaining power levels of the selected subsetof beams for generating a first factor (block 1120). The power levelsmay be calculated by the wireless device 310 or through cloud computing,for example. The first factor corresponds to W₁, for example.

Then, method 1100 obtains phases of the selected subset of beams forgenerating a second factor, wherein the first factor and the secondfactor are part of the parameters of the precoder (block 1130). Thephases may be calculated by the wireless device 310 or through cloudcomputing. The second factor corresponds to W₂. And the precoder w isgiven by: w=W₁W₂.

The parameters of the precoder are then sent to the network node 320 ina CSI feedback report, for example. It should be noted that theparameters of the precoder sent to the network node 320 may alsocomprise the rotation factors and other information.

In some embodiments, the selected subset of beams are single polarizedbeams, corresponding to transmission on a single polarization. In someother embodiments, the subset of beams is selected in polarizationpairs, each polarization pair corresponding to a dual polarized (DP)beam.

In some embodiments, the selection of the subset of beams is done bydetermining beams which have the largest wideband received power.

When calculating the wideband received power, the wireless device 320actually calculates the power coefficients or power levels. The powercoefficients of the subset of beams can be expressed as a first matrixwhich corresponds to W₁=B_(I) _(S) √{square root over (P)}. The powerlevels (or power coefficients or powers) are selected or obtained on awideband basis (corresponding to the first frequency granularity), forexample. Additionally, the power levels can be obtained to be the same(or common) to all layers of a multi-layer transmission, meaning thatthe beam power levels are shared between all layers and polarizations.The phases can be obtained to be specific to each layer of themulti-layer transmission, for example, meaning that the phases areassignable independently for each individual layer.

Furthermore, the selected subset of beams can be sorted in order ofpower levels or power strength. Also, a first beam that is of lessstrength than a second beam can be quantized with a coarser quantizationthan that of the second beam. As such, the number of bits can be savedwhen reporting the parameters of the precoder to the network node. Itshould be noted that the first beam having less strength than the secondbeam means that the power level of the first beam is inferior to thepower level of the second beam.

In order to reduce the CSI feedback overhead, the powers of the selectedbeams can be quantized at a first quantization resolution and the phasesof the selected beams can be quantized at a second quantizationresolution. In order to further reduce the CSI feedback overhead, anindex of a strongest beam (e.g. having the highest power level) of theselected subset of beams is specified and the rest of the beams in theselected subset is specified in an unordered fashion with regards tostrength, in the report to the network node. Also, the power levels canbe uniformly quantized between a first value and a second value, thefirst value being a minimum power level.

In some embodiments, the first factor is generated for a first frequencygranularity, and the second factor is generated for a second frequencygranularity, the first frequency granularity corresponding to an entirefrequency bandwidth (of a carrier) and the second frequency granularitycorresponding to a frequency subband within the frequency bandwidth.

In some embodiments, for a precoder of rank 2, the phases of theselected subset of beams for a first layer is a function of the phasesof the selected subset of beams for a second layer.

It should be noted that the terms “power level”. “power coefficient” and“amplitude” are interchangeably used in this disclosure to characterizethe beams which comprise an amplitude/power level and a phase.

FIG. 12 illustrates a flow chart of a method 1200 for determiningtransmission parameters in a wireless communication system, such as 300,according to an embodiment. The method is performed by a network node,such as 320, for example.

Method 1200 starts with block 1210 by, responsive to transmittingreference signals to the wireless device, receiving precoder parameterswhich include a subset of beams selected from a plurality of orthogonalbeams and power levels of the selected subset of beams, for a firstfrequency granularity, and phases of the selected subset of beams for asecond frequency granularity. The reference signals may comprise CSI-RS,RS, or any other signals that allow to determine a channel estimate.

Method 1200 continues with determining the transmission parameters basedon the received precoder parameters (block 1220). For example, based onthe received information, the network node determines the transmissionparameters, such as a modulation coding scheme and a precoding schemefor the data transmission of the wireless device. Based on the receivedinformation, the network node can decide/choose to use the precodersuggested by the wireless device or it can decide/choose to use anotherprecoder. The network node then sends the determined transmissionparameters to the wireless device for data transmission.

FIG. 13 illustrates a flow chart of a method 1300 for determiningtransmission parameters in a wireless communication system, such as 300,according to another embodiment. The method is performed by a networknode, such as 320, for example.

Method 1300 starts with block 1310 by, responsive to transmittingreference signals to the wireless device, receiving precoder parameterswhich include a subset of beams selected from a plurality of orthogonalbeams, a first factor associated with power levels of the selectedsubset of beams, and a second factor associated with phases of theselected subset of beams. The reference signals may comprise CSI-RS, RS,or any other signals that allow to determine a channel estimate.

Method 1300 continues with determining the transmission parameters basedon the received precoder parameters (block 1320). For example, based onthe received information, the network node determines the transmissionparameters, such as a modulation coding scheme and a precoding schemefor the data transmission of the wireless device. Based on the receivedinformation, the network node can decide/choose to use the precodersuggested by the wireless device or it can decide/choose to use anotherprecoder. The network node then sends the determined transmissionparameters to the wireless device for data transmission.

FIG. 14 is a block diagram of a base station 320, such as an eNodeB,configured to determine transmission parameters based on informationreceived from the wireless device, according to some embodiments. Thebase station 320 has processing circuitry 1410 having a memory 1450 anda processor 1440. The base station 320 further comprises a networkinterface 1430 and one or more transceivers 1420. In some embodiments,the transceiver 1420 facilitates transmitting wireless signals to andreceiving wireless signals from wireless device 310 (e.g., via anantenna), the one or more processors 1440 executes instructions toprovide some or all of the functionalities described above as beingprovided by the network node 320, the memory 1450 stores theinstructions for execution by the one or more processors 1440, and thenetwork interface 1430 communicates signals to backend networkcomponents, such as a gateway, switch, router, Internet, Public SwitchedTelephone Network (PSTN), core network nodes or radio networkcontrollers, etc. The network interface 1430 is connected to theprocessor and/or memory.

As an example, the processor 1440 is configured to perform methods 1200and 1300. The one or more processors 1440 may include any suitablecombination of hardware and software implemented in one or more modulesto execute instructions and manipulate data to perform some or all ofthe described functions of the network node 320, such as those describedin methods 1200 and 1300. In some embodiments, the one or moreprocessors 1440 may include, for example, one or more computers, one ormore central processing units (CPUs), one or more microprocessors, oneor more applications, one or more application specific integratedcircuits (ASICs), one or more field programmable gate arrays (FPGAs)and/or other logic. In certain embodiments, the one or more processors1440 may comprise one or more of the modules discussed below withrespect to FIG. 18. It should be noted that the processing circuitry1410 is similar to processing circuitry 900. The processor 1440 issimilar to processor 920 and the memory 1450 is similar to memory 910.

The memory 1450 is generally operable to store instructions, such as acomputer program, software, an application including one or more oflogic, rules, algorithms, code, tables, etc. and/or other instructionscapable of being executed by one or more processors 1440. Examples ofmemory 1450 include computer memory (for example, Random Access Memory(RAM) or Read Only Memory (ROM)), mass storage media (for example, ahard disk), removable storage media (for example, a Compact Disk (CD) ora Digital Video Disk (DVD)), and/or or any other volatile ornon-volatile, non-transitory computer-readable and/orcomputer-executable memory devices that store information.

FIG. 15 illustrates an example wireless device 310 configured todetermine precoder parameters in a wireless communication system, suchas 300.

The wireless device 310 includes an antenna 1520, radio front-endcircuitry 1530, processing circuitry 1510, a computer-readable storagemedium 1540, an input interface 1560 and output interface 1570. Antenna1520 may include one or more antennas or antenna arrays, and isconfigured to send and/or receive wireless signals, and is connected toradio front-end circuitry 1530. The radio front-end circuitry 1530 maycomprise various filters and amplifiers, is connected to antenna 1520and processing circuitry 1510, and is configured to condition signalscommunicated between antenna 1520 and processing circuitry 1510. Incertain alternative embodiments, UE 310 may not include radio front-endcircuitry 1530, and processing circuitry 1510 may instead be connectedto antenna 1520 without radio front-end circuitry 1530.

In some embodiments, the processing circuitry 1510 may comprise aprocessor 1580 and a memory such as the storage/memory 1540, theprocessor 1580 being connected to the input and output interfaces 1560and 1570. The memory 1540 contains instructions which, when executed bythe processor, configure processor to perform the one or more functionsdescribed in method 1000 of FIG. 10 and 1100 of FIG. 11, for example.The processing circuitry 1510 is similar to 700 of FIG. 7.

Processing circuitry 1510 may comprise and/or be connected to and/or beadapted for accessing (e.g., writing to and/or reading from) memory1540. Such memory 1540 may be configured to store code executable bycontrol circuitry and/or other data. e.g., data pertaining tocommunication, e.g., configuration and/or address data of nodes, etc.Processing circuitry 1510 may be configured to control any of themethods described herein and/or to cause such methods to be performed,e.g., by the processor. Corresponding instructions may be stored in thememory 1540, which may be readable and/or readably connected to theprocessing circuitry 1510. The memory 1540 is similar to memory 1450 ofFIG. 14.

Antenna 1520, radio front-end circuitry 1530, processing circuitry 1510,and/or input interface 1560 and output interface 1570 may be configuredto perform any transmitting operations described herein as beingperformed by a wireless device. Any information, data and/or signals maybe transmitted to a network node and/or another wireless device. Theinput interface 1560 and output interface 1570 can be collectivelyreferred to as a network interface, which is connected to the processorand/or memory.

FIG. 16 is a block diagram of an example embodiment of a wireless device310, according to another embodiment, the wireless device 310 configuredto determine parameters of a precoder in a wireless communicationsystem. The wireless device 310 includes a selecting module 1610, afirst obtaining module 1620 and a second obtaining module 1630. Theselecting module 1610 is configured to select a subset of beams from aplurality of orthogonal beams (e.g. corresponding to the columns of thebeam space transformation matrix). The first obtaining module 1620 isconfigured to obtain power levels of the selected subset of beams forgenerating a first factor. The second obtaining module 1630 isconfigured to obtain phases of the selected subset of beams forgenerating a second factor. The wireless device 310 may further includea transmitting module (not shown) configured to transmit/send theprecoder parameters to a base station, or network node.

FIG. 17 is a block diagram of an example embodiment of a wireless device310, according to another embodiment, the wireless device 310 configuredto send parameters of a precoder to a network node, in a wirelesscommunication system. The wireless device 310 includes a first sendingmodule 1710, and a second sending module 1720. The first sending module1710 is configured to send to the network node a subset of beamsselected from a plurality of orthogonal beams and power levels of theselected subset of beams, for a first frequency granularity. The secondsending module 1720 is configured to send phases of the selected subsetof beams for a second frequency granularity. In some embodiments, thefirst and second modules can be combined to form one sending module.

FIG. 18 is a block diagram of an example embodiment of a network node320, such as an eNB or base station, according to another embodiment,the network node 320 configured to determine transmission parameters fora wireless device, in a wireless communication system. The network node320 comprises a receiving module 1810, and a determining module 1820.

The receiving module 1810 is configured to receive parameters of aprecoder, in response to transmitting reference signals to a wirelessdevice. The precoder parameters may include a subset of beams selectedfrom a plurality of orthogonal beams and power levels of the selectedsubset of beams for a first frequency granularity, and phases of theselected subset of beams for a second frequency granularity. Theprecoder parameters may also include a subset of beams selected from aplurality of orthogonal beams, a first factor associated with powerlevels of the selected subset of beams, and a second factor associatedwith phases of the selected subset of beams.

The determining module 1820 is configured to determine transmissionparameters based on the received precoder parameters.

Further exemplary embodiments are given below:

Embodiment 1

A method at a wireless device for determining parameters to enableconstruction of a precoder codebook structure in a wirelesscommunication system, the method comprising: selecting a subset ofcolumns of a beam space transformation matrix, each column correspondingto a single polarized, SP, beam, each SP beam having a phase and anamplitude; and factoring each column into at least two factors, a firstfactor having a first frequency granularity and a least a second factorhaving a second frequency granularity.

Embodiment 2

The method of Embodiment 1 wherein a first factor is an amplitude of anSP beam and a second factor is a phase of an SP beam.

Embodiment 3

The method of Embodiment 2, wherein the amplitude of an SP beam isquantized at a first quantization resolution; and a phase of the SP beamis quantized at a second quantization resolution.

Embodiment 4

The method of Embodiment 1, wherein the columns are selected inpolarization pairs of columns, each polarization pair corresponding to adual polarized, DP, beam.

Embodiment 5

The method of Embodiment 1, further comprising sorting the SP beams inorder of power strength and coarsely quantizing a first SP beam that isof less strength than a second SP beam.

Embodiment 6

The method of Embodiment 1, wherein the first granularity is appliedacross an entire frequency bandwidth and the second granularity is afunction of frequency subbands within the frequency bandwidth.

Embodiment 7

The method of Embodiment 1, further comprising transmitting the factorsand the frequency granularities to a base station.

Embodiment 8

A wireless device for determining parameters to enable construction of aprecoder codebook structure in a wireless communication system, thewireless device comprising: processing circuitry including a memory anda processor, the memory configured to store precoder information, theprecoder information including frequency granularities of factors ofsingle polarized, SP, beams; the processor configured to: select asubset of columns of a beam space transformation matrix, each columncorresponding to an SP beam; each SP beam having a phase and anamplitude; and factor each column into at least two factors, wherein afirst factor has a first frequency granularity and a least a secondfactor has a second frequency granularity.

Embodiment 9

The wireless device of Embodiment 8, wherein a first factor is anamplitude of an SP beam and a second factor is a phase of an SP beam.

Embodiment 10

The wireless device of Embodiment 9, wherein the amplitude of an SP beamis quantized at a first quantization resolution; and a phase of the SPbeam is quantized at a second quantization resolution.

Embodiment 11

The wireless device of Embodiment 8, wherein the columns are selected inpolarization pairs of columns, each polarization pair corresponding to adual polarized, DP, beam.

Embodiment 12

The wireless device of Embodiment 8, further comprising sorting the SPbeams in order of power strength and coarsely quantizing a first SP beamthat is of less strength than a second SP beam.

Embodiment 13

The wireless device of Embodiment 8, wherein the first granularity isapplied across an entire frequency bandwidth and the second granularityis a function of frequency subbands within the frequency bandwidth.

Embodiment 14

The wireless device of Embodiment 8, further comprising a transmitterconfigured to transmit the factors and the frequency granularities to abase station.

Embodiment 15

A wireless device for determining parameters to enable construction of aprecoder codebook structure in a wireless communication system, thewireless device comprising: a memory module configured to store precoderinformation, the precoder information including frequency granularitiesof factors of single polarized, SP, beams: a column selection moduleconfigured to select a subset of columns of a beam space transformationmatrix, each column corresponding to an SP beam; each SP beam having aphase and an amplitude; and a factorization module configured to factoreach column into at least two factors, wherein a first factor has afirst frequency granularity and a least a second factor has a secondfrequency granularity.

Embodiment 16

A base station for determining transmission parameters for transmissionto a wireless device based on information received from the wirelessdevice, the base station comprising: processing circuitry including amemory and a processor; the memory configured to store precoderinformation: the processor configured to determine a rank indicator,modulation and coding scheme based on the precoder information: atransmitter configured to transmit the rank indicator, modulation andcoding scheme to the wireless device; and a receiver configured toreceive, from the wireless device, precoder information including: asubset of columns of a beam space transformation matrix, each columncorresponding to a signal polarized, SP, beam, the SP beams havingphases and amplitudes; and frequency granularities of factors of the SPbeams.

Embodiment 17

A precoder codebook comprising precoders for channel state information,CSI, feedback in a wireless communication system, the precoders in thecodebook comprising: a weighted sum of multiple orthogonal beamsselected from a rotated two dimensional discrete Fourier transform. DFT,and where an amplitude and a phase of a beam are separated withdifferent frequency granularities.

As will be appreciated by one of skill in the art, the conceptsdescribed herein may be embodied as a method, data processing system,and/or computer program product. Accordingly, the concepts describedherein may take the form of an entirely hardware embodiment, an entirelysoftware embodiment or an embodiment combining software and hardwareaspects all generally referred to herein as a “circuit” or “module.”Furthermore, the disclosure may take the form of a computer programproduct on a tangible computer usable storage medium having computerprogram code embodied in the medium that can be executed by a computer.Any suitable tangible computer readable medium may be utilized includinghard disks, CD-ROMs, electronic storage devices, optical storagedevices, or magnetic storage devices.

Some embodiments are described herein with reference to flowchartillustrations and/or block diagrams of methods, systems and computerprogram products. It will be understood that each block of the flowchartillustrations and/or block diagrams, and combinations of blocks in theflowchart illustrations and/or block diagrams, can be implemented bycomputer program instructions. These computer program instructions maybe provided to a processor of a general purpose computer (which thenforms a special purpose computer), special purpose computer, or otherprogrammable data processing apparatus to produce a machine, such thatthe instructions, which execute via the processor of the computer orother programmable data processing apparatus, create means forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks.

These computer program instructions may also be stored in a computerreadable memory or storage medium that can direct a computer or otherprogrammable data processing apparatus to function in a particularmanner, such that the instructions stored in the computer readablememory produce an article of manufacture including instruction meanswhich implement the function/act specified in the flowchart and/or blockdiagram block or blocks.

The computer program instructions may also be loaded onto a computer orother programmable data processing apparatus to cause a series ofoperational steps to be performed on the computer or other programmableapparatus to produce a computer implemented process such that theinstructions which execute on the computer or other programmableapparatus provide steps for implementing the functions/acts specified inthe flowchart and/or block diagram block or blocks.

It is to be understood that the functions/acts noted in the blocks mayoccur out of the order noted in the operational illustrations. Forexample, two blocks shown in succession may in fact be executedsubstantially concurrently or the blocks may sometimes be executed inthe reverse order, depending upon the functionality/acts involved.Although some of the diagrams include arrows on communication paths toshow a primary direction of communication, it is to be understood thatcommunication may occur in the opposite direction to the depictedarrows.

Computer program code for carrying out operations of the conceptsdescribed herein may be written in an object oriented programminglanguage such as Java® or C++. However, the computer program code forcarrying out operations of the disclosure may also be written inconventional procedural programming languages, such as the “C”programming language. The program code may execute entirely on theuser's computer, partly on the user's computer, as a stand-alonesoftware package, partly on the user's computer and partly on a remotecomputer or entirely on the remote computer. In the latter scenario, theremote computer may be connected to the user's computer through a localarea network (LAN) or a wide area network (WAN), or the connection maybe made to an external computer (for example, through the Internet usingan Internet Service Provider).

Many different embodiments have been disclosed herein, in connectionwith the above description and the drawings. It will be understood thatit would be unduly repetitious and obfuscating to literally describe andillustrate every combination and subcombination of these embodiments.Accordingly, all embodiments can be combined in any way and/orcombination, and the present specification, including the drawings,shall be construed to constitute a complete written description of allcombinations and subcombinations of the embodiments described herein,and of the manner and process of making and using them, and shallsupport claims to any such combination or subcombination.

It will be appreciated by persons skilled in the art that theembodiments described herein are not limited to what has beenparticularly shown and described herein above. In addition, unlessmention was made above to the contrary, it should be noted that all ofthe accompanying drawings are not to scale. A variety of modificationsand variations are possible in light of the above teachings.

1. A method for sending parameters of a precoder from a wireless device to a network node, in a wireless communication system, the method comprising: sending, to the network node, a subset of beams selected from a plurality of orthogonal beams and power levels of the selected subset beams, for a first frequency granularity; and sending, to the network node, phases of the selected subset beams, for a second frequency granularity, wherein the selected beams, the power levels and the phases of the selected subset beams are part of the parameters of the precoder.
 2. The method of claim 1, wherein the first frequency granularity corresponds to an entire frequency bandwidth and the second granularity corresponds to a frequency subband within the frequency bandwidth.
 3. The method of claim 1, wherein the power levels are the same for all layers of a multi-layer transmission and the phases are specific to each individual layer of the multi-layer transmission.
 4. The method of claim 1, wherein the parameters of the precoder are sent in a Channel State Information (CSI) feedback report to the network node.
 5. The method of claim 1, wherein the selected subset of beams corresponds to transmission on a single polarization.
 6. The method of claim 1, wherein the selected subset of beams is selected in polarization pairs, each polarization pair corresponding to a dual polarized (DP) beam.
 7. The method of claim 1, wherein the selected subset of beams is selected by determining beams which have a largest wideband received power.
 8. The method of claim 1, further comprising sorting the selected subset of beams in order of power strength and quantizing a first beam that is of less strength than a second beam.
 9. The method of claim 1, wherein the power levels of the selected subset of beams are quantized at a first quantization resolution and the phases of the selected beams are quantized at a second quantization resolution.
 10. The method of claim 9, further comprising sending the quantized power levels and quantized phases to the network node.
 11. The method of claim 1, wherein sending the selected subset of beams comprises sending indices corresponding to the selected subset of beams.
 12. (canceled)
 13. (canceled)
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 28. A wireless device for sending parameters of a precoder to a network node, in a wireless communication system, the wireless device comprising a processing circuitry configured to cause the wireless device to: send, to the network node, a subset of beams selected from a plurality of orthogonal beams and power levels of the selected subset beams, for a first frequency granularity; and send, to the network node, phases of the selected subset beams, for a second frequency granularity, wherein the selected beams, the power levels and the phases of the selected subset beams are part of the parameters of the precoder.
 29. The wireless of claim 28, wherein the processing circuitry comprises a processor, a memory and a network interface both connected to the processor, the memory containing instructions that, when executed, cause the processor to perform the operations of sending the selected subset of beams with the power levels and sending the phases of the selected subset of beams.
 30. The wireless of claim 28, wherein the first frequency granularity corresponds to an entire frequency bandwidth and the second granularity corresponds to a frequency subband within the frequency bandwidth.
 31. The wireless device of claim 28, wherein the power levels are the same for all layers of a multi-layer transmission and the phases are specific to each individual layer of the multi-layer transmission.
 32. The wireless device of claim 28, wherein the selected subset of beams corresponds to transmission on a single polarization.
 33. The wireless device of claim 29, wherein the processor is configured to select the subset of beams in polarization pairs, each polarization pair corresponding to a dual polarized, DP, beam.
 34. The wireless device of claim 29, wherein the processor is configured to select the subset of beams by determining beams which have a largest wideband received power.
 35. The wireless device of claim 29, wherein the processor is configured to sort the selected subset of beams in order of power strength and to quantize a first beam that is of less strength than a second beam.
 36. The wireless device of claim 29, wherein the processor is configured to quantize at a first quantization resolution the power levels of the selected subset of beams and to quantize at a second quantization resolution the phases of the selected subset of beams.
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 54. A method for determining transmission parameters in a wireless communication system, the method comprising: responsive to transmitting reference signals to the wireless device, receiving precoder parameters which include a subset of beams selected from a plurality of orthogonal beams and power levels of the selected subset of beams, for a first frequency granularity, and phases of the selected subset of beams for a second frequency granularity; and determining the transmission parameters based on the received precoder parameters.
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 56. A network node for determining transmission parameters in a wireless communication system, the network node comprising a processing circuitry configured to cause the network node to: responsive to transmitting reference signals to the wireless device, receive precoder parameters which include a subset of beams selected from a plurality of orthogonal beams and power levels of the selected subset of beams, for a first frequency granularity, and phases of the selected subset of beams for a second frequency granularity; and determine the transmission parameters based on the received precoder parameters.
 57. The network node of claim 56, wherein the processing circuitry comprises a processor, a memory and a network interface both connected to the processor, the memory containing instructions that, when executed, cause the processor to perform the operations of receiving and determining.
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